Residual oil hydrodesulfurization process by catalyst pretreatment and ammonia addition

ABSTRACT

A process for hydrodesulfurizing an asphaltic oil in the presence of a catalyst comprising Group VI and Group VIII metals on alumina wherein the catalyst is presulfided with a low molecular weight organic sulfur compound such as carbon disulfide in the absence of hydrogen and wherein ammonia is added to the process.

United States Patent Brunn et al. 1 Jan. 7, 1975 15 1 RESIDUAL OILHYDRODESULFURTZATION 3,308,054 3/1967 Duir et al 208/211 PROCESS BYCATALYST PRETREATMENT 3x32; V 252/439 van enrooy AND AMMONIA ADDHTION3,562,800 2/1971 Carlson et a1 208/216 [751 Inventors: Louis W. Brunn;James A. Frayer; 3,717,571 2 1973 Schulman 208/210 John A. Paraskos;Stephen J. Yanik, all of Pittsburgh, Pa. g I Primary lzxammer-Delbert E.Gantz 1 1 Asslgnee: Gulf Research & Development Assistant E.\'aminer--G..1. Crasanakis Company, Pittsburgh, Pa.

[22] Filed: Jan. 22, 1974 21] Appl. No.: 434,584 1 1 B T T A process forhydrodesulfurizing an asphaltic oil in the U-S- Clpresence of a atalystomprising Group and s i Group on is [581 Fleld of Search 208/216 21presulfided with a low molecular weight organic sulfur 208/11 1; 252/439compound such as carbon disulfide in the absence of hydrogen and whereinammonia is added to the pro- [56] References Cited Cess UNITED STATESPATENTS 3,284,344 11/1966 Demeester et a1 208/216 14 Claims, 10 DrawingFigures SECOND STAGE 153,111111111111 e No NH; OR

TEMPERATURE F 31m E r58 2 670 l l l I BOTH CS2 PRETREATMENT AND 1,000PPM AMMONIA CS PRETR EAT MENT ElTHER CS PRETREATMENT OR AMMONIA ADDITION(BUT NOT BOTH) l l l l l l CATALYST PAIENTED JAN H975 SHEET 1 IIF 8HYDRODESULFURIZATION OF 650 F.+( 343C.,+) RESIDUAL OIL FEED 0.55 LHSV.

0.88 LHSVI mI3 X QMUDQOmm 10 modzmDm kzwummm .Iw m3 INCREASINGTEMPERATURE INCREASING TIME 'PATENTEDJAN' W5 3.859.204

SHEET 2 BF 8 GAS OIL FEED FIRST STAGE I I I I I l-IJ I D E n: 790- g 2 1(42|c.) s z 3 u 2 O I- 8 z 780-- Q I (45C.) 2

770 I I (mom) 2 6 l0 l4 I8 22 CATALYST AGEI DAYS FIRST STAGE (CATALYSTVQLUME=|I SECOND STAGE (CATALYST VOLUME=2) BOO (42ft) I I I I SECONDSTAGE N0 NH3, 0.5 LHSV'\ Ta0- u. (416C) 3 o I I? 1eo :3 (404C) FIRST NH5 Y LHSV g 140- 5 (393C) u P v 720- (sea-c) I I I I CATALYST AGE! DAYSPATENIEDJANY 1% 3,859,204

SHEEI 7 0F 8 HYDROGEN AND CON AMINANT GASES FIG. 9

RESIDUAL OIL HYDROGEN AND CONTAMINANT GAS s 6 2 H2 NH TEMPERATURE-"FPATENTED 3,859,204

SHEET 8 OF 8 FIG. /0

SECOND STAGE 730 (388m I I I I I I T I 25% NO NH3 OR CS PRETREATMENTEITHER 700* -cs PRETREATMENT (3WD) 0R AMMQNIA ADDITION (BUT NOT BOTH)69o I (366C) BOTH cs PRETREATMENT M AND L000 PPM AMMONIA 570 l l I -l 1'(354m 0 2 -3 4 5 s 7 a 9 IO [2 CATALYST AGE: DAYS RESIDUAL OILHYDRODESULFURIZATION PROCESS BY CATALYST PRETREATMENT AND AMMONIAADDITION This invention relates to a catalytic hydrodesulfurizationprocess for removing sulfur from crude or residual petroleum oils, orfrom synthetic oils such as oils from tar sands, shale and coal, whereinammonia or an ammonia precursor compound is injected into the reactionzone.

The present invention is particularly directed towards thehydrodesulfurization of a crude or asphaltic residual oil such as anatmospheric or vacuum reduced petroleum crude which contains theasphaltene fraction of the crude from which it is derived. The nickel,vanadium and sulfur content of the liquid charge can vary over a widerange. For example, for an atmospheric reduced crude nickel and vanadiumcan comprise 0.002 to 0.03 weight percent (20 to 300 parts per million)or more of the feed oil while sulfur can comprise about 2 to 6 weightpercent of the oil. These are examples only and for some atmospheric andvacuum distilled crude residues the metals and sulfur levels can behigher. The feed oil generally contains nitrogen, a portion of which isconverted to ammonia in situ. However, the injection of extraneousnitrogen over a fresh catalyst at start-of-run (SOR) in accordance withthe present invention provides sufficient ammonia upstream in thecatalyst bed to inhibit and moderate progressive coking of the catalystbed with catalyst age, which coking otherwise would progressivelyproceed from the top downwardly through the catalyst bed. Normally,because of the low rate of nitrogen removal in residual oilhydrodesulfurization,sufficient ammonia is not present in the system toinhibit such coking.

The added ammonia or ammonia precursor com pound results inchemisorption of ammonia on the strong and weak acid sites of thecatalyst, with greatest chemisorption occurring on the strong acidsites. It is an unexpected feature of this invention that ammoniachemisorption on the hydrodesulfurization catalyst inhibitshydrocracking activity and resulting coke forma tion without inhibitingdesulfurization activity. Since hydrocracking activity anddesulfurization activity are both dependent upon the acid sites on thecatalyst support, it is unexpected that neutralizing these acid sitesdepresses one type of reaction utilizing these sites without depressingthe other type of reaction also utilizing these sites.

A further unexpected feature of this invention is that controlledammonia chemisorption causes the catalyst aging rate to be reduced oreven stablized by reducing the hydrogen flow rate, within limits, ascompared to a higher hydrogen flow rate. It is a highly unusualphenomenon of the present invention that process improvement can beachieved by reducing hydrogen flow rate when hydrotreating processes arealmost universally improved by increased rather than decreased hydrogenflow rates. Another view of this unusual hydrogen flow rate phenomenonis that controlled levels of a relatively small amount of ammoniaadvantageously displace a relatively large amount of hydrogen, with thedisplacement advantage actually being lost if excessive hydrogen ispresent, which results in a low concentration of ammonia in the gas. Areason that hydrogen rate cannot be excessive is that the concentrationof ammonia and hydrogen sulfide in the hydrogen are both im- Ill portantand reduction in hydrogen rate can advanta geously increase ammoniaconcentration in the hydrogen gases.

The hydrodesulfurization process of this invention employs conventionalresidual oil hydrodesulfurization reaction conditions such as, forexample, a hydrogen partial pressure of at least 750 or 1,000 pounds persquare inch (52.5 or kg/cm generally, 1,000 to 5,000 pounds per squareinch (70 to 350 kglcm pref erably, and 1,000 to 3,000 pounds per squareinch (70 to 210 kglcm more preferably. It is the partial pressure ofhydrogen rather than total reactor pressure which determineshydrodesulfurization activity. Therefore, the hydrogen stream should beas free of other gases as possible and contaminated hydrogen gases areflashed from the first stage effluent followed by addition of freshhydrogen to the flash residue when two hydrodesulfurization reactorstages are employed in se- The gas circulation rate can be between about2,000 or 3,000 and 20,000 standard cubic feet per barrel (36 or 54 to360 SCM/lOOL), generally, preferably about 3,500 to 10,000 standardcubic feet per barrel (63 to 180 SCM/lOOL), or most preferably 3,500 toless than 7,400 standard cubic feet per barrel (63 to 133 SCM/l00L) ofgas preferably containing 85 percent or more of hydrogen. The mol ratioof hydrogen to oil can be between about 4:1 and :1. Reactor temperaturescan range between about 600 or 650 and 900F. (316 or 343 and 482C),generally, and between about 690 and 800F. (365and 427C), preferably.The temperature should be low enough so that with a 650F.+ (343C.+) feedoil, less than about 10, 15 or 20 percent of the feed oil which boilsabove 650F. (343C.) will be cracked to furnace oil or lighter, i.e. tomaterials boiling below 650F. (343C.). The liquid hourly space velocityin any reactor of this invention can be between about 0.2 and 10,generally, and between about 0.3 and l or 1.25, preferably.

The catalyst employed in the process comprises sulfided Group VI andGroup VIII metals on a noncracking support such as alumina with thesubstantial absence of cracking components. Conventional Group V] andGroup VIII metal combinations can be employed. Preferred catalysts arenickel-cobaltmolybdenum or cobalt-molybdenum on alumina. Examples ofother metal combinations are nickeltungsten and nickel-molybdenum. Thesupport is a non-cracking alumina which is essentially free of silica,and which contains less than 1 weight percent silicon, generally, andpreferably less than 0.5 or 0.1 weight percent of silicon.

Because of the non-cracking nature of the catalyst, the processconditions of the present invention provide a product oil (with orwithout ammonia addition) comprising more than 40 or 50 or more than 80,or percent by weight of material having a boiling point greater than theinitial boiling point of the feed oil. Very little hydrocracking beyondwhat is required for desulfurization occurs in accordance with thepresent invention and therefore the hydrogen consumption will begenerally in the range of only 150 to 1,500 and preferably in the rangeof 300 to 1,200 standard cubic feet per barrel of feed (2.7 to 27 andpreferably 5.4 to 21.6 SCM/ L). The feed to the hydrodesulfurizationreactor will generally have an initial boiling point of not less than375F. (191C.) and will preferably have an initial boiling point of atleast 620 or 650F. (327 or 343C.).

Thus, the amount of material obtained from the process whose boilingpoint is lower than the initial boiling point of the feed oil is lessthan 50 or 60 percent, generally, or preferably less than 10, 15 or 20weight percent. A feed having an initial boiling point greater than650F. (343C.) e.g. vacuum tower bottoms having an initial boiling pointin the range of 750F. to 900F. (399 to 482C.) or more may be employed inthe process of the present invention. In that event, the amount ofmaterial obtained from the second hydrodesulfurization zone whoseboiling point is below 650F. (343C.) will be less than 10 or 20 weightpercent.

Tests were conducted to illustrate the ability of anickel-cobalt-molybdenum on alumina catalyst of this invention tochemisorb ammonia. Tests were also conducted to illustrate the ammoniachemisorption ability of a similar catalyst after it had first beenimpregnated with sodium to reduce its acidity in advance of ammoniachemisorption. The characteristics of these catalysts are shown in thefollowing table.

Na Promoted NiCoMo NiCoMo on on Alumina Alumina Nickel 0.5 Cobalt l 01.0 Molybdenum 8.0 8.0 Silicon 0.27 0.27 Sodium 0 09 0.09 Iron 0.02 0.02Alumina (l) (1) Physical inspections:

Compacted Density,g/cc 0.69 0.72 Surface area, mlg I94 I81 Pore volume,cc/g 0.52 0.52 Pore size distribution,

100-300 A radius l4 l4 50-100 A radius 50 54 30-50 A radius 28 26 7-30 Aradius 8 6 Surface acidity NH;

adsorption. meg/g,F:

350 (177C.) 0.62 0.44 400 (204C.) 0.55 0.38 500 (260C.) 0.43 0.285 600(3l6C.) 0.34 0.21 700 (371C.) 0.27 0.16 800 (427C.) 0.21 0.12 900(482C.) 0.l6 0.09

(l) Difference The above data show that an alumina supported catalyst oflow acidity, even after its acidity is partially quenched by sodiumimpregnation, still retains sufficient acidity to chemisorb asignificant quantity of ammonia. The data show that the quantity ofammonia adsorbed in both catalysts changes significantly withtemperature increase, with the amount of ammonia adsorbed on eachcatalyst being relatively great at low temperatures and decreasing withincreasing temperature. It is therefore an important feature of thisinvention that ammonia injection is employed in a process to produce aproduct of defined sulfur level requiring reaction temperature toprogressively increase with catalyst age to compensate for catalystdeactivation. In this manner, the catalyst acid sites are provided withchemisorbed ammonia before temperatures for significant hydrocrackingare reached at SOR so that both strong and weak acid sites, butprimarily strong acid sites, are provided with chemisorbed ammonia. Ascatalyst aging progresses and reaction temperature increase, an in- 5creasing quantity of ammonia is desorbed and this desorption occursinitially most easily from the weak acid sites and subsequently fromstronger acid sites, providing progressively stronger catalyst sites forreaction purposes as catalyst age increases. Thereby, relatively weakcatalyst sites are employed at start-of-run (SOR) and relatively strongcatalyst sites are employed at endof-run (EOR). Therefore, the strongeracid sites which are most apt to coke are not made available until thelater stages of a catalyst cycle.

The advantage of ammonia injection in accordance with the presentinvention is due to interaction of ammonia with the alumina catalystsupport but interaction of ammonia with the active hydrogenation metalson the alumina support to possibly form a metal ammonia complex ispresumed to be detrimental to the hydrodesulfurization reaction andshould be avoided entirely or to the greatest extent possible. Reactionof ammonia with the Group VI and Group VIII metals on the catalystsupport will be avoided in part in accordance with this invention eitherby presulfiding these metals prior to process start-up or prior toammonia addition. Presulfiding can occur in the presence of ammonia andhydrogen while maintaining a high hydrogen sulfide to ammonia ratio witha sulfur-containing gas oil or furnace oil sulfiding agent. Sincepresulfided Group VI and Group VIII metal sulfides tend to lose sulfurduring a hydrotreating process if sufficient hydrogen sulfide is notpresent in the gas phase, it is also important that sufficient hydrogensulfide is generated during the hydrodesulfurization process so that theaverage hydrogen sulfide concentration along the length of the reactionzone is equal to or greater than the average ammonia concentration alongthe length of the reaction zone. For example, if the hydrogen sulfideconcentration at the inlet and outlet of a reaction zone is about 0 and7,000 parts per million based on feed oil, respectively, the averageammonia concentration in the reaction zone must be less than about 3,500parts per million. (At a second reaction zone inlet, it is usual for anamount such as 1,000 parts per million of hydrogen sulfide to bedissolved in the feed oil as the result of first stage desulfurization.)By maintaining a greater average concentration of hydrogen sulfide thanammonia in the reactor gases equilibrium is shifted towards the metalsulfide, which is active for desulfurization, instead of towards a metalammonia complex. While the sulfide is the preferred active state of thecatalyst metals, the reaction product of the catalytic metals andammonia is considered to be relatively disadvantageous to thehydrodesulfurization reaction.

While it is relatively easy to maintain high hydrogen sulfide level in afirst hydrodesulfurization stage, particular caution is required toinsure that hydrogen sulfide concentration is maintained above ammoniaconcentration in the second or third reactors of a series ofhydrodesulfurization reactors, because these reactors hydrodesulfurize apreviously desulfurized oil after flashing off gaseous reaction productsand therefore treat a feed oil having a low concentration of sulfur,which sulfur is of a relatively refractory nature as compared to thatprocessed in the first reactor of the series. In the reactor systemcomprising a series of reactors, it is advantageous to remove byflashing the impure hydrogen from the first reactor effluent and chargefresh hydrogen to the second reactor in series in order to provide ahigh hydrogen partial pressure in the second reactor. If the hydrogensulfide dissolved in the oil feed to the second reactor plus thehydrogen sulfide generated in the second reactor is insufficient tomaintain the catalyst in the second reactor in a sulfided state and isinsufficient to compete with the ammonia in the second reactor gaseousphase for reaction with the catalyst metals, it may be necessary tointroduce hydrogen sulfide to the second reactor from an externalsource. Whether all of the hydrogen sulfide in the second reactor isgenerated in situ, or some very small amount of hydrogen sulfide isadded to the second reactor from an external source, the averageconcentration of hydrogen sulfide in the second reactor must be abovethe average ammonia concentration in the second reactor.

Since the hydrogen sulfide concentration in a hydrodesulfurizationreactor is lowest near the reactor inlet and highest near the reactoroutlet, it is particularly advantageous to add only a portion of thetotal ammonia to the reactor inlet and to add the remaining portion ofthe total ammonia at one or more positions down stream in the reactor.Multiple ammonia addition tends to provide zones of increasing ammoniaconcentration corresponding to zones of increasing hydrogen sulfideconcentration and tends to avoid an excess in ammonia concentrationrelative to hydrogen sulfide concentration near the reactor inlet whichwould lower catalyst activity at the reactor inlet.

It is apparent from the above that the present invention is especiallyapplicable to a hydrodesulfurization reaction because the presence ofhydrogen sulfide generated in the process modulates the adverse effectof ammonia in the process. Therefore, in accordance with the presentinvention the amount of ammonia added is limited by the desireddesulfurization reaction so that the concentration of ammonia in thereactor gases is less than the concentration of hydrogen sulfide in thereaction gases but is sufficiently high so that the averageconcentration of hydrogen sulfide in the reactor gases is increased bythe improved desulfurization due to the addition of the ammonia.

FIG. I illustrates the acidic behavior of a freshnickel-cobalt-molybdenum on alumina catalyst as reflected by change inmeasured production of 375 to 650F. (191 to 343C.) furnace oil duringsingle stage hydrodesulfurization of an oil which boils above thefurnace oil range. The oil is a 650F.+ (343C.+) atmospheric towerbottoms of a Kuwait petroleum crude oil. The desulfurization reduced thesulfur content of the oil from 4 to 1 weight percent without ammoniainjection. FIG. 1 shows that at start-of-run at each of the two spacevelocities tested a relatively elevated amount of by-product furnace oilis produced. As reaction time and temperature increase, the amount offurnace oil produced decreases (although not indicated the decreaseoccurs over a throughput of feed oil of about 1 or 2 barrels of oil perpound of catalyst) until a minimum quantity of furnace oil is produced.The decrease in furnace oil production is probably caused by a coatingof acid sites of the catalyst with coke to irreparably destroy (absentcombustion regeneration) the acidic nature of these sites. Thereupon, asreaction temperature continues to increase, furnace oil productionsteadily increases from its minimum valve due to thermal (non-catalytic)cracking. FIG. 1 illustrates that an acid capacity existed in the freshcatalyst but, in the absence of ammonia injection, particularly in theabsence of very early ammonia injection prior to passage of l or 2barrels of oil per pound of catalyst, the acidic capacity is rapidlydestroyed due to coke deposition and thereafter a further increase intemperature induces thermal (as contrasted to catalytic) cracking. Thisobservation indicates that when employing the ammonia injection methodof this invention in a hydrodesulfurization process utilizingprogressive temperature increases to compensate for catalyst aging thebest mode of reactor temperature control is to precede an incrementaltemperature increase with an incremental increase in added ammonia levelto preclude coke formation due to the increase in temperature. Ammoniainjection prior to coke formation induces only a temporary loss ofcatalyst acidity which is recovered in the same catalyst cycle upon asubsequent temperature increase to desorb ammonia from the catalystsurface, but coke formation prior to ammonia injection induces a loss incatalyst acidity which cannot be reversed by temperature increase and isapparently permanent for at least the same catalyst cycle. As the cycleproceeds and elevated temperatures are attained coke will no doubt blocksome of the acid sites present on the catalyst and therefore a lowerammonia level might be required because coke has substantially reducedthe acidity in spite of the coke inhibiting effect of ammonia. At thispoint, a reduction in the level of ammonia addition may be required topermit sufficient activity of the active sites still available.

All data of FIG. 1 were taken in desulfurizing the feed oil from 4 to Iweight percent. Since at point A more furnace oil was produced than atpoint B, the extent of curve segment AB indicates the extent of excesscatalyst acidity in terms of excess furnace oil yield beyond thatrequired to accomplish the desired degree of desulfurization. Similarly,the extent of curve segment CD indicates the extent of excess catalystacidity in terms of excess furnace oil yield beyond that required toaccomplish the desired degree of desulfurization. Curves segments BE andDE indicate generally furnace oil produced by thermal rather thancatalytic cracking activity.

In accordance with the present invention, ammonia is injected into thehydrodesulfurization process in an amount sufficient to quench excessinitial catalyst acidity beyond that required to accomplish the desireddegree of desulfurization, which excess catalyst activity tends tocatalytically produce furnace oil and coke during initialhydrodesulfurization. Because the sulfur removed in a second serieshydrodesulfurization stage is more refractory and is more deeplyimbedded in asphaltene molecules than the sulfur removed in the firststage, this excess catalyst acidity tends to produce coke in a secondseries hydrodesulfurization catalyst stage. The amount of ammonia addedshould not be sufficient to depress catalyst acidity to an extent thatthe ability of the catalyst to accomplish desulfurization is overlyinhibited. In general, the amount of ammonia injected, based on bothfeed oil and on the concentration of ammonia in the reactor hydrogenatmosphere, should be established so that a given degree ofdesulfurization can be accomplished with some process advantage, such asa greater total amount of sulfur removal within the temperature limitsof the process or at a particular total throughput of feed oil per pound(gram) of catalyst, a lower temperature at a given catalyst age andproduct sulfur level, a higher space velocity, a smaller catalystrequirement to produce a product having a given sulfur level or a lowerhydrogen partial pressure. Ammonia injection, by depressing cokeformation at any given throughput of oil per pound (gram) of catalyst,permits a lengthy cycle life in a second or subsequent series catalyststage in which coke formation is considerably more serious than in afirst catalyst stage. Therefore, ammonia injection can provide a secondor subsequent series catalyst cycle life of 4 to 50, generally, or 10 to40 barrels of feed oil per pound of catalyst (0.0014 to 0.017 or 0.0035to 0.014 m /g). First stage cycle life is as great or greater thansecond stage cycle life.

The utility of ammonia injection is considerably greater in a second ora third hydrodesulfurization reactor than in a first reactor with whichit is in series since a first hydrodesulfurization reactor is notdeactivated as rapidly as a second or third reactor with which it is inseries. Coke deposition is considerably lower in a first reactor than ina second reactor. The life of the catalyst in a first reactor is moreseverely limited by metals deposition on the catalyst than by cokeformation, while the life of the catalyst in a second or third seriesreactor is more severely limited by coke deposition than by metalsdeactivation. Since the coke laydown in second and subsequent seriesreactors is much greater than in the first reactor, the amount ofcatalyst of a similar composition required at a given space velocity isgenerally considerably greater (by a factor of 50 or 100 percent, ormore) in a second series reactor than in a first reactor with which itis in series, even though the absolute amount of sulfur removed in thesecond reactor is less than in the first reactor.

One reason that the removal of sulfur is more difficult in a secondseries stage than in a first series stage is that the concentration ofsulfur available for reaction is lower in the second stage feed (firststage effluent) than in the first stage feed. If a firsthydrodesulfurization stage reduces oil sulfur level from 4 to 1 weightpercent at one liquid hourly space velocity, the reaction rate constantat a given temperature is proportional to (l/Csp I/CSF) where, g V H V VI CS1 is the concentration of sulfur in the product oil,

n a Cgr is the concentration of sulfur in the feed oil. The reactionrate constant in desulfurization of Kuwait atmospheric bottoms in afirst reactor I stage is given approximately as Because the sulfurconcentration is much smaller in the second stage oil stream than in thefirst stage oil stream, in order to expend the same amount of chemicalwork in the second stage to further reduce the sulfur concentration froma level of l to 0.3 weight percent, which is a much smaller absolutequantity of sulfur removal, about one-third the space velocity, or 3times the quantity of catalyst at the same space velocity, should beemployed in the second stage on the basis of second order kinetics toaccomplish the same amount of chemical work which was expended in thefirst stage. This is illustrated in the following calculation:

1/03 1/1 znsy pas LH SV 0.33 Therefore, in this example either threetimes as much catalyst is required in the second stage as compared tothe first stage or the second stage space velocity must be one-third thefirst stage space velocity when utilizing the same quantity of catalystin two stages, or operation at a significantly higher catalyst averagetemperature is required.

A further reason that the chemical work or energy expended in a secondresidual oil hydrodesulfurization reactor stage is greater than in afirst hydrodesulfurization reactor stage with which it is in series isthat inherently the less refractory sulfur in the feed oil is removed inthe first stage leaving the more refractory sulfur for removal in thesecond stage. The most refractory sulfur in residual oil is concentratedin asphaltene molecules. The first stage tends to remove sulfur close tothe periphery of the asphaltene molecules, such as sulfur in alkylgroups projecting from the polycondensed aromatic ring nucleus of theasphaltene molecules so that minimum hydrocracking and, therefore,minimum coke formation is required to reach and remove these lowrefractory sulfur atoms. In contrast, second stage sulfur removal tendsto penetrate to removal of sulfur imbedded within the polycondensedaromatic ring nucleus of asphaltene molecules so that more extensivehydrocracking and, therefore, more extensive coke formation is requiredto accomplish molecular penetration to these more refractory sulfuratoms.

The criticality of utilizing ammonia injection in a second stage of aresidual oil hydrodesulfurization process which removes highlyrefractory sulfur, as compared to its use in a first stage with which itis in series in which less refractory sulfur is removed, is illustratedin the accompanying figures. The criticality of utilizing ammoniainjection in a residual oil hydrodesulfurization process as contrastedto a distillate oil hydrodesulfurization process is also shown in thefigures. Because of the relative absence of polycondensed aromatic ringmolecules in distillate oils, the sulfur contained in distillate oils ismuch less refractory than the sulfur contained in asphaltene-containingresidual oils. It is well known that catalyst cycle durations indistillate oil hydrodesulfurization processes are much less limited bycoke formation than are catalyst cycle durations in residual oilhydrodesulfurization processes generally enabling lower hydrogen partialpressure operation for the relatively cleaner gas oil feedstocks.

FIG. 2 presents the aging curve in a process for percenthydrodesulfurization of a 650 to 1,050F. (343 to 565C.) heavy gas oilcontaining 2.29 weight percent sulfur at 675 psig (40 kg/cm with anickelcobalt-molybdenum on aluminia hydrodesulfurization catalyst. Thecurve shows that catalyst aging is so slight that it was not a problemprior to ammonia injection. In direct contrast to the effect of ammoniain residual oil hydrodesulfurization catalyst aging, ammonia injectionin the range employed in the residue process tests in distillate gas oilhydrodesulfurization triggered severe catalyst aging. This indicatesthat strong catalyst acid sites on the alumina catalyst support areapparently required in distillate oil hydrodesulfurization at therelatively low hydrogen partial pressure employed therein. Thus itappears that gas oil desulfurization cannot operate with the benefitsobserved in residue hydrodesulfurization if the ammonia levels arecomparable. Thus, a significantly lower rate of ammonia injection,probably 100 parts per million, or less, based on fresh feed might beemployed beneficially in distillate oil hydrodesulfurization.

FIG. 3 presents aging curves without ammonia addition for the first andsecond stages in series of a residual oil hydrodesulfurization processwith an interstage flashing step to remove contaminant gases. Curve Frepresents the first stage aging characteristics of anickel-cobalt-molybdenum on alumina catalyst in desulfurizing a 650F.+(343C.+) Kuwait residual oil from 4 to 1 weight percent sulfur at l LHSVwith a reactor metallurgy constraint temperature of 790F. (421C). Afterflashing of hydrogen containing gaseous impurities and subsequentaddition of fresh hydrogen, the flash residue of the first stagehydrodesulfurization step is hydrodesulfurized in a second stage toreduce its sulfur content from 1 to 0.3 weight percent. Curve Grepresents the second stage aging curve. The weight of catalyst employedin the second stage was twice that employed in the first stagereflecting the greater chemical work expended in the second stage due tothe lower concentration of sulfur and the more refractory nature of thesulfur being removed. Even though a total of 3 weight percent sulfur wasremoved in the first stage and a total of only 0.7 weight percent sulfurwas removed in the second stage and even though the second stageutilized twice as much catalyst as the first stage, FIG. 3 shows thatthroughout the catalyst aging cycle a considerably higher temperaturewas required in the second stage as compared to the first stage due to amuch greater coke deposit on the second stage catalyst as compared tothe first stage catalyst.

Because of the considerably more severe catalyst aging problem in thesecond stage as compared to the first stage due to greater cokedeposition on the second stage catalyst, a considerably larger volume ofcatalyst is required in the second stage as compared to the first stage.Therefore, the method of this invention can be particularly directed toinjection of ammonia into the second stage of a residual oilhydrodesulfurization process to reduce the catalyst volume requirementin the second stage and to thereby increase the space velocity capacityin the second stage to a level closer to that prevailing in the firststage, so that increased balance is imparted to a series multistagehydrodesulfurization reactor system.

Although it is usual for catalyst requirements to be greater in a secondstage than in a first series stage, aside for ammonia injection anddepending upon the relative desulfurization severities in the stages thecatalyst requirement can be greater in the first stage than in thesecond stage. In this instance, ammonia injection into the first stagewould tend to equalize the space velocity requirements in the twostages. For example, with a 53 percent Kuwait reduced crude whose sulfurcontent is reduced from 4 to 1 weight percent in a first stage at lLHSV, and whose sulfur content is further reduced to 0.1, 0,3, 0.5 and0.8 weight percent, respectively, in a second stage with an interstageflashing step, the respective second stage space velocity requirementsare as follows:

Second First Second Stage Stage Stage Product LHSV LHSV Sulfur As aprogressively deeper degree of sulfur removal occurs in the secondstage, the sulfur becomes increasingly more difficult to remove. In ausual system, a space time of about 1.0 is required to remove 3 weightpercent sulfur from feed oil in a first stage while a space time ofabout 2.0 is required to remove only 0.7 weight percent sulfur from thefeed oil in a second stage. Therefore the efficiency of the catalystutilization becomes lower with greater depth of sulfur removal. Thisefficiency may be referred to as Space Time Efficiency for residual oilhydrodesulfurization. Ammonia injection increases the efficiency ofrefractory sulfur removal. Catalyst efficiency for the two stages can bedefined as follows:

In a three reactor series system, the respective space velocitiesrequired to reduce a feed oil having 4 percent sulfur to 1 percentsulfur in a first stage, to 0.3 percent sulfur in a second stage andfinally to 0.1 percent sulfur in a third stage are:

First Stage: 1 volume catalyst/volume oil/hour to remove 3 percent ofthe sulfur Second Stage: 2 volume catalyst/volume oil/hour to remove 0.7percent of the sulfur Third Stage: 2 volume catalyst/volume oil/hour toremove 0.2 percent of the sulfur The efficiency as defined above foreach stage is:

First Stage: 3/1 3 Second Stage: 0.7/2 0.35

Third Stage: 0.2/2 0.1 The overall efficiency for the three stages is3.9/5 or 0.78.

The use of ammonia injection into the second and third stages to reducecatalyst requirements so that the same amount of catalyst is employed ineach stage would provide a catalyst efficiency in each stage as follows:

First Stage: 3/1 3 Second Stage: 0.7/1.0 0.7

Third Stage: 0.2/1.0 0.2 The overall efficiency for the three reactorsystem employing ammonia injection is 3.9/3 or 1.3. It is therefore seenthat the use of ammonia injection considerably increases catalystefficiency in a residual oil hydrodesulfurization process.

FIG. 4 shows the results of tests made to illustrate the effect ofammonia injection rate in terms of ratio of ammonia to feed oil in thesecond series reactor of a residual oil hydrodesulfurization processemploying a nickel-cobalt-molybdenum on alumina hydrodesulfurizationcatalyst. Curve K is the second stage catalyst aging curve in a testconducted without ammonia injection. Curve K has no indication oftemperature stabilization, i.e. a relatively constant temperature withincreasing catalyst age indicating catalyst coke level is stabilized.Curves L, M and N are catalyst aging curves for the second stage wheninjecting 1,250, 1,500 and 5,000 ppm of nitrogen as ammonia,respectively. FIG. 4 shows that as the ammonia addition rate increased,the required start-of-run reaction temperature to accomplish a fixeddegree of desulfurization disadvantageously increased, so there was nostart-of-run advantage in utilizing elevated levels of ammoniainjection. These start-of-run data tend to indicate that ammoniainjection is disadvantageous. However, after seven or eight days curve Nhas temperature stabilized, indicating that with advancing catalyst agean additional advantage appears due to ammonia injection. Thistemperature stabilization with advancing age disadvantageously occurs incurve N at an elevated temperature. Furthermore, curve N shows thatammonia injection at start-of-run inhibits desulfurization as comparedto lower ammonia injection rates. On the other hand, although in curve Ma trend is not yet sharply defined, curve M shows incipientstabilization at a lower temperature than in curve N. At the lowerammonia injection level of curve L there is no indication of temperaturestabilization. It is apparent from FIG. 4 that to obtain a high level ofadvantage from ammonia injection, the ammonia to oil ratio and thecatalyst age must be correlated. With the feed oil and test conditionsof FIG. 4, in order to obtain temperature stabilization with ammoniainjection, more than 1,250 and preferably at least 1,500 but less than4,000 or 5,000 parts per million of nitrogen based on feed oil arerequired. In addition, the catalyst must be aged somewhat depending uponthe conditions employed in the run. The initial aging period prior tostabilization may be avoided by pretreatment of the catalyst withammonia. For example, when the catalyst is presulfided with asulfurcontaining gas oil or furnace oil and hydrogen, ammonia can beadded to the presulfiding step. Catalyst presulfiding occurs within thereactor and upon start-up of residual oil flow with hydrogen and ammoniamore stable initial activity may be realized. FIG. 4 shows that whencatalyst stability is achieved, the presence of ammonia permits therequired product sulfur level to be achieved with smaller incrementaltemperature increases with catalyst age (a lower aging rate) than whenthe same process is operated without ammonia addition.

A highly important aspect of this invention is shown in FIG. 4, at curveregions N, S and T. Curve region N shows that when 5,000 parts permillion of nitrogen are added, the required reaction temperature atstart-ofrun is greater than the required start-of-run reactiontemperature of curve M where 1,500 parts per million nitrogen as ammoniawere added under otherwise the same conditions. The ammonia level of5,000 parts per million exceeded the averaage concentration of hydrogensulfide which was about 3,500 parts per million of sulfur as hydrogensulfide, based on feed in'the reaction zone. The significance ofinsufficient hydrogen sulfide in relation to ammonia is even morestrongly illustrated by curve T. Curve T represents a period of anextremely rapid rate of catalyst aging. As indicated in FIG. 4, therapid aging rate of curve T was triggered by stopping oil flow andpassing hydrogen only over the catalyst without either oil, hydrogensulfide or ammonia. The flow of hydrogen over the catalyst apparentlyresulted in a severe loss of sulfur and ammonia from the catalyst,whereby catalyst metals were converted to metal hydrides. When feed oilwas returned to the reaction system together with hydrogen and 4,000parts per 5 million of ammonia under conditions prevailing earlier,

rapid aging curve T resulted. It appears that the hydrogen treatmentresulted in a catalyst so devoid of sulfur and ammonia that the ammoniasubsequently added successfully competed with the hydrogen sulfidegenerated in situ to react with the Group V] and Group VIII metals onthe catalyst to render these metals substantially inactive fordesulfurization, or possibly the ammonia was not as quickly adsorbed onthe catalyst surface to prevent or inhibit coking as it had been doingduring curve segment S, or some combination of these reasons.

If the catalyst is presulfided with gas oil and/or furnace oil withhydrogen, the presence of ammonia during the presulfiding step may tendto limit coking and reduce the SOR aging rate of the catalyst. Thefollowing comparative tests involving sulfiding with carbon disulfide atrelatively low temperature and pressure, in one case with hydrogen andin another case with nitrogen in place of hydrogen, show a reduced cokecontent in the sulfided catalyst when employing nitrogen in place ofhydrogen.

cs Sulfiding of NiCoMo/ M 0 Catalyst Test 1 Test 2 CS, Pressure (atom)Sulfiding Temp.F

0.1 Nitrogen 0.5 Hydrogen The stabilization feature of curve segment Sindicates that even with a presulfided catalyst it is important for theaverage ammonia concentration in the reaction zone to be less than theaverage hydrogen sulfide concentration in the reaction zone so thatthroughout the catalyst cycle hydrogen sulfide can successfully competewith ammonia in reacting with catalyst metals. To further illustrate theimportance of relatively high hydrogen sulfide concentrations relativeto ammonia concentrations, curves N, M and L represent conditionswherein hydrogen sulfide concentrations are progressively relativelyhigher than the ammonia concentration resulting in a decrease in SORtemperatures and aging stabilization temperatures (where stabilizationoccurred). Therefore, the data of FIG. 4 show that the ammoniaconcentration should be sufficiently high to improve catalyst agingcharacteristics to an extent to permit either a reduction in catalystrequirements, a reduction in hydrogen requirements per barrel of feedoil, a reduction in hydrogen partial pressure, or so as to permit agreater amount of sulfur to be removed from the feed oil with a givenweight of catalyst over a catalyst cycle, but the average ammoniaconcentration in the reactor should not be greater than the averagehydrogen sulfide concentration.

FIG. 5 outlines further data taken to illustrate ammonia injection intoa second series residual oil hydrodesulfurization stage. The catalystemployed in the tests was nickel-cobalt-molybdenum on aluminapresulfided with carbon disulfide and the reactor reduced residue sulfurlevel from 1 to 0.3 weight percent, unless otherwise indicated in FIG.5. The reactor was operated at 0.5 LHSV, 2,120 psig (148 kg/cm and 4,250standard cubic feet per barrel of hydrogen (76.5 SCM/lOOL). Testconditions deviating from these conditions are indicated in FIG. 5.Curve indicates an aging run conducted without ammonia injection whilecurve P indicates an aging run conducted with 1,000 ppm of nitrogeninjection as ammonia based on feed oil at start-of-run.

Referring to the ammonia injection aging run illustrated by curve P ofFIG. 5, it is seen that stable activity was attained after 5 days ofoperation at 0.5 Ll-ISV with a temperature requirement of only695F.(366C.) for a 0.3 weight percent sulfur product with 1,000 ppm ofnitrogen injected. Curve 0, also representing a test made at 0.5 LHSV toproduce 0.3 weight percent sulfur product, shows that with no ammoniaaddition, the temperature requirement after 5 days is 706F. (374C.) andthe aging rate does not stabilize but rather continues at about F.(09C.) per day.

Stable operation was also attained in curve P at 0.9 LHSV and 729F.(387C.) while producing 0.3 percent sulfur product with 1,000 ppm ofnitrogen injected based on feed oil. Under more severe conditions, a0.12 weight percent sulfur product was attained at conditions of 0.44Ll-ISV and 745F. (396C). Under these conditions, the aging rate wasmoderate, i.e. about 0.6F. (0.36C.) per day.

Aging at a rate of about 3F. (18C.) per day was experienced insubsequent operation after days as an apparent result of desorbingammonia from the catalyst during an earlier higher temperatureoperation. However, the aging was arrested upon increasing the rate ofammonia addition from 1,000 ppm N to 1,500 ppm N.

FIG. 6 shows the results of second stage aging runs with anickel-cobalt-molybdenum on alumina catalyst at 1.0 LHSV, 2,120 psig(148 kglcm and 4,600 standard cubic feet per barrel of hydrogen (83SCM/lOOL), unless otherwise indicated in FIG. 6. Curve Q shows theresults of an aging run without ammonia injection while curve R showsthe results of an aging run with ammonia injection. To take advantage ofthe improvement due to ammonia injection, an increased feed rate of 1.0LHSV was employed in curve R as compared to a space velocity of 0.5 LHSVfor curve Q. Initially, a 10F. (5.5C.) temperature elevation wasrequired at the higher space velocity compared to the base test. After13 days, the temperature elevation requirement was only about 2F.(12C.). However, activity failed to stabilize. Subsequent increases inammonia addition from 1,250 ppm N to 1,800 ppm on the 24th day, 2,100ppm N on the th day and 2,300 ppm N on the 38th day were required tomoderate the aging rate. Each step-wise increase in ammonia additionresulted in temporarily stabilizing the activity, but at a highertemperature. However, the overall aging rate over this period was about1F. (06C.) per day which is about the same as that obtained in the basetest without ammonia at the lower space velocity.

A decrease in the reactor gas rate from 4,600 SCF/B (83 SCM/IOOL) to4,150 standard cubic feet per barrel (75 SCM/IOOL) on the 4th day had noapparent effect on catalyst activity. A further decrease to 3,700standard cubic feet per barrel (67 SCM/IOOL) on the 30th day, however,appeared to trigger aging at a rate of about 2F. (1.2C.) per day. Agingappeared to be arrested by restoring the gas rate to 4,150 standardcubic feet per barrel SCM/l 00L). These observations indicate that thegas rate as well as the concentration of ammonia have an effect uponcatalyst aging rate.

FIG. 7 contains data which further illustrate the effect of ammoniaconcentration in the hydrogen gas. FIG. 7 shows a first stage catalystaging curve wherein ammonia injection is employed to reduce sulfur levelin a Kuwait reduced crude residual oil from 4 weight percent to 1 weightpercent in the presence of a nickelcobalt-molybdenum on alumina catalystat 0.88 LHSV, 2,050 psig (144 kglcm and 7,400 standard cubic feet perbarrel of hydrogen (133 SCM/lOOL), unless otherwise indicated in FIG. 7.As shown in FIG. 7, aging was abruptly arrested after 28 days ofoperation when ammonia addition (as aniline) was held constant at 1,500ppm N while the hydrogen gas rate was reduced from 7,400 to 3,500standard cubic feet per barrel (133 to 63 SCM/L). The effect of thischange was to increase the ammonia concentration in the reactor gas, ata fixed ammonia injection rate based on feed oil, and also to increasehydrogen sulfide concentration. Subsequent reduction of the gascirculation rate to 3,000 standard cubic feet per barrel (54 SCM/lOOL),however, appears to trigger loss of aging stability, indicating that theadvantage of optimum ammonia concentration cannot be achieved at theexpense of an excessively reduced hydrogen rate so that the hydrogen gasrate should be greater than 3,000, 3,100 or 3,200 standard cubic feetper barrel (54, 56 or 58 SCM/lOOL).

FIG. 7 illustrates a highly unusual effect achieved by the ammoniainjection method of this invention in that the catalyst aging rate isstabilized by a reduction in hydrogen feed rate. The reduction inhydrogen rate per se probably has an adverse effect on the process, butthe adverse effect is apparently more than offset by the advantageachieved by the increase in ammonia concentration in the gas and by theincrease in hydrogen sulfide concentration in the gas, since, as notedabove, the hydrogen sulfide concentration must be greater than theammonia concentration. A process advantage ensuing on a reduction inhydrogen rate is highly unusual in the face of the almost universallyobserved phenomenon in hydrotreating processes that a decrease inhydrogen rate tends to inhibit the hydroprocessing reaction. The showingof FIG. 7 indicates that as hydrogen concentration in the reactor gasesincreases for any reason, eg due to addition to the reactor of adownstream hydrogen quench, it becomes advantageous to inject only aportion of the ammonia requirement at the reactor inlet and to injectadditional ammonia downstream with the hydrogen quench in order tomaintain optimum ammonia concentration in the reactor gases at everyregion of the reactor. Another important indication of the data of FIG.7 is that the process can be controlled at least in part by reducinghydrogen throughput rate at a given ammonia injection rate therebyincreasing ammonia concentration which in turn inhibits the catalystaging rate independently of reaction temperature.

In accordance with this invention, ammonia precursors which are rapidlyconverted to ammonia under reaction conditions can be injected into ahydrodesulfurization reactor in place of ammonia itself. Examples ofsuitable ammonia precursorsare aniline, pyridene, quinoline, pyrolle,amines,-etc.

The process of this invention is illustrated schematically in FIG. 8. Asshown in FIG. 8, reactors l and 12 each contain three spaced-apart fixedbeds of Group VI and Group VIII metal on alumina catalyst particles.Crude or residual oil containing asphaltenes is charged through line 14while hydrogen is charged through line 16. Ammonia or a compound whichforms ammonia under reaction conditions is injected into hydrogen line16 through line 18. Hydrogen and feed oil are passed downwardly throughthe fixed catalyst beds under hydrodesulfurization conditions to removethe less refractory sulfur in the oil and reduce the sulfur content inthe oil from 4 weight percent to 1 weight percent. Reactor is smallerthan reactor 12 and contains less catalyst, when reactors 10 and 12contain the same catalyst, since it removes the less refractory sulfurfrom the feed oil and because the sulfur it removes is relatively highin concentration.

Effluent is removed from reactor 10 through line 20 and passed to flashchamber 22 wherein gases comprising hydrogen sulfide, ammonia, lighthydrocarbons and hydrogen are flashed overhead through line 24, leavingan oil residue which is removed through line 26 and passed downwardlythrough second series reactor 12 together with hydrogen entering throughline 28. Ammonia is charged to the hydrogen stream through line 30.Refractory sulfur is removed in reactor 12 wherein the oil sulfurcontent is reduced from I to 0.3 weight percent, or lower. Product isremoved from reactor 12 through line 32.

A preferred embodiment of this process is illustrated schematically inFIG. 9. As shown in FIG. 9, hydrodesulfurization reactors 40 and 42 aredisposed in series with each other. Reactor 40 would normally be smallerthan reactor 42 for high level desulfurization of a high sulfurchargestoc-k and contains less catalyst, when reactors 40 and 42 containthe same catalyst composition, since it removes the relatively lessrefractory sulfur from the feed oil and since the sulfur-it removes ispresent in the feed oil at a relatively high concentration. Thehydrodesulfurization catalyst employed in each reactor is comprised ofGroup VI and Group VIII metal on alumina. Reactor 40 containsspaced-apart fixed beds of catalyst 44, 46 and 48 while reactor 42contains spaced-apart fixed beds of catalyst 50, 52 and 54.

Crude or residual feed oil containing asphaltenes is charged to reactor40 through line 56 while hydrogen is charged through line 58. Ammonia oran ammonia precursor is charged to hydrogen line 58 through line 60.Additional hydrogen for temperature quenching together with ammonia isadded between the catalyst beds, first through lines 62 and 64 and thenthrough lines 66 and 68. Ammonia is added to the downstream quenchhydrogen streams in order to regulate ammonia concentration in reactorgases as additional hydrogen is added without necessitating an undulyhigh ammonia concentration in the reactor inlet. The less refractorysulfur is removed in reactor 40 and the relatively high sulfurconcentration in the oil is reduced from 4 to 1 weight percent.

Effluent is removed from reactor 40 through line 70 and passed to flashchamber 72 from which a gaseous stream comprising hydrogen sulfide,ammonia, light hydrocarbons and hydrogen is removed through line 77. Oilresidue is removed from flash chamber 72 through line 76 and passed toreactor 42 which is larger than and contains more catalyst than reactor40.

Hydrogen is charged to the top of reactor 42 through line 78 and ammoniaor an ammonia precursor is injected into hydrogen line 78 through line80. Quench hydrogen and ammonia are charged to reactor 42 betweencatalyst beds through lines 82 and 84 and 86 and 88. The amount ofammonia injected into the quench hydrogen streams regulates ammoniaconcentration in the reaction gases downstream in the reactor withoutnecessitating an unduly high ammonia concentration in the hydrogen gasesat the reactor inlet. Product is removed from reactor 42 througheffluent line 90.

The process of FIG. 9 is designed on the basis of the showing in FIG. 4that an excessive ammonia concentration at the reactor inlet creates aneed for an unduly high start-of-run reactor temperature prior to andafter reaching temperature stabilization in the process and also in viewof the showing in FIG. 7 that there is an optimum concentration ofammonia in the reactor gases. Therefore, the process of FIG. 9 permitsoptimization of ammonia concentration in the reactor gases at thereactor inlet, avoiding excessive ammonia concentration at the reactorinlet, and also prevents occurrence of insufficient ammoniaconcentration downstream in the reactor due to both addition oftemperature quench hydrogen and to formation of hydrocarbon gases in thereactor.

Ammonia injection exhibits a synergistic advantage when employed in ahydrodesulfurization system wherein the catalyst has been presulfidedwith a low molecular weight organic sulfide, such as carbon disulfide,in the absence of hydrogen. In catalyst presulfiding tests under thisprocedure, carbon disulfide dissolved in furnace oil was fed over a bedof catalyst at a temperature of 400F. (204C.) and a pressure of 200 psig(l4 kg/cm in the absence of hydrogen in sufficient quantity and over alength of time to provide the equivalent of 0.10 pounds (45.4 grams) ofsulfur per pound (454 grams) of catalyst.

When sulfiding a catalyst with higher molecular weight organic sulfides,the presence of hydrogen is required for extracting the sulfur from theorganic mole cule and for the catalyst sulfiding reaction to proceed.

' It is a feature of the low molecular weight organic sulfidescontemplated herein that they sulfide the catalyst directly without theintervention of hydrogen. It is critical that hydrogen not be addedduring the carbon disulfide catalyst pretreatment because the hydrogenmay react with the carbon disulfide or any other low molecular weightorganic sulfide which is used to produce hydrogen sulfide and ahydrocarbon. The organic sulfide employed must be of sufficiently lowmolecular weight to sulfide the catalytic metals directly and in theabsence of added hydrogen and hydrogen sulfide. The presence of hydrogenis conducive to the formation of metal hydrides on the catalyst inaddition to metal sulfides. Metal hydrides can be detrimental since theyare uncontrollably more active than metal sulfides and tend tohydrocrack excessively and build up coke. Excessive acidity in thecatalyst opposes the catalyst acidity depressant effect of ammonia.

Suitable presulfiding conditions with low molecular weight organicsulfides include temperatures of to 700F. (66 to 371C.) and pressures offrom one atmosphere to 500 psig (35 kglcm Suitable low molecular weightorganic compounds which can presulfide hydrodesulfurization catalystswithout the addition of hydrogen include organic sulfides and mercaptanscontaining less than 4, 6 or 8 carbon atoms such as dimethyl sulfide,ethyl sulfide, diethyl sulfide, dipropyl sulfide, dibutyl sulfide,methylethyl sulfide and ethylpropyl sulfide.

For comparative purposes, one catalyst was presulfided with carbondisulfide without hydrogen and within the test conditions indicatedabove and a similar catalyst was sulfided conventionally by contact witha sulfur-containing gas oil and hydrogen at a temperature of 660F.(349C) and a pressure of 2,120 psig (148 kg/cm The conventional catalystpresulfiding method employs a sulfur containing gas oil and hydrogen,both of which materials are present in the hydro desulfurization processitself so that the conventional presulfiding operation would inherentlyoccur in situ in the course of the hydrodesulfurization process. Incontrast, the low molecular weight organic sulfide presulfiding methodwould not inherently occur in situ in the hydrodesulfurization processsince the low molecular weight organic sulfide is not present infeedstocks of the present process and since the hydrodesulfurizationprocess always has hydrogen gas in the reactor atmothe use of bothtogether produced a 22F. (13C.) temperature advantage, which is greaterthan the sum of the temperature advantage of each effect by itself. Moreimportantly, the combination of carbon disulfide pretreatment andammonia injection resulted in a stabilized catalyst aging rate afteronly 6 days, whereas when each effect was utilized by itself;astabilized catalyst aging rate had not occurred after 1 1 days,indicating that new coke is formed on the catalyst at a faster rate thenit is removed.

The following table presents data illustrating the savings innickel-cobalt-molybdenum catalyst requirement in a two-stage residualoil hydrodesulfurization process with an interstage flash of contaminantgases over a 6- month catalyst cycle wherein 650F.+ (343C.+) reducedKuwait crude is reduced in sulfur content in a first stage from 4 to 1weight percent sulfur and then further reduced after an interstage flashin a second stage to either a 0.3 weight percent or 0.1 weight percentsulfur level. The savings illustrated is achieved by CS without hydrogenpretreatment of the catalyst and ammonia injection into the second stageonly. The table shows that a considerable savings in hydrogenconsumption accompanies CS pretreatment and ammonia injection.

Product Sulfur Level, Wt.%

CS, Pretreatment and Ammonia injection Overall Relative CatalystRequirements EOR H, Consumption, SCF/B (SCM/lOO sphere. The lowmolecular weight organic sulfide is not present in thehydrodesulfurization process because it boils below the initial boilingpoint of the hydrodesul furization feed oil.

FIG. 10 shows the results of second stage residual oilhydrodesulfurization tests when charging the effluent from a first stagehydrodesulfurization reactor containing one weight percent sulfur tofurther reduce its sulfur content to a level of 0.3 weight percentutilizing progressively increasing temperatures with catalyst age andunder reactor conditions including 2,120 psig 148 kg/cm 0.5 LHSV and4,250 standard cubic feet per barrel of hydrogen (77 SCM/lOOL). Curve1-! shows an aging run conducted without ammonia injection and withoutcarbon disulfide catalyst pretreatment. The catalyst pretreatmentutilized sulfur-containing gas oil and hydrogen. Curve 1 shows the agingresults obtained when carbon disulfide pretreatment without ammoniainjection was employed and when ammonia injection was employed withoutcarbon disulfide catalyst pretreatment (but with gas oil and hydrogenpresulfiding). Substantially the same aging curve was obtained in eachcase. Curve J is the aging curve obtained in a test employing bothammonia injection and carbon disultide without hydrogen pretreatment ofthe catalyst.

Curve 1 indicates the occurrence of a synergism when both carbondisulfide pretreatment and ammonia injection are utilized together, ascontrasted to the use of each by itself and without the other. FIG. 10shows that at nine days of catalyst age, the use of each alone produceda 9F. (54C.) temperature advantage, while The above data show thatoverall catalyst requirements in a two-stage residual oilhydrodesulfurization process are reduced at least one-third or even atleast 40 percent, while overall hydrogen consumption at a given catalystage to reduce feed sulfur level by a particular amount can be reduced 5or 10 percent or more by carbon disulfide catalyst pretreatment andammonia injection into the second reactor stage. Generally, in atwo-stage process, the employment of carbon disulfide without hydrogencatalyst pretreatment and ammonia injection can reduce catalystrequirements at least 5 or 10 percent, and as much as 20 or 30 percent,or more. Hydrogen consumption can be reduced at least 1, 2 or 3 or atleast 5, 10 or 15 percent, at a given sulfur removal level.

We claim:

1. A process for the hydrodesulfurization of an asphaltene-containingoil at a reaction temperature in the range 600 to 900F. wherein said oiland hydrogen are passed over a sulfided catalyst containing Group VI andGroup Vlll metals on alumina, said catalyst having been sulfided by anorganic sulfur compound containing less than 8 carbon atoms in theabsence of hydrogen, adding to said catalyst ammonia in an amount toincrease hydrodesulfurization as compared to the absence of saidammonia, the ammonia concentration in the reaction gases being lowerthan the average hydrogen sulfide concentration in said gases, andincrementally increasing the temperature within said temperature rangeto compensate for loss of catalyst activity due to catalyst aging.

2. The process of claim 1 wherein said organic sulfur compound containsless than 6 carbon atoms.

3. The process of claim 1 wherein said organic sulfur compound containsless than 4 carbon atoms.

4. The process of claim 1 wherein said organic sulfur compound is carbondisulfide.

5. The process of claim 1 wherein said organic sulfur compound has aboiling point below the boiling range of said oil.

6. The process of claim 1 wherein the ratio of said ammonia to said oilis changed during said process.

7. The process of claim 1 wherein the ratio of said ammonia to said oilis changed incrementally prior to said incremental changes intemperature.

8. The process of claim 1 wherein less than percent of said oil isconverted to material boiling below the initial boiling point of saidoil.

9. The process of claim 1 wherein less than 10 percent of said oil isconverted to material boiling below the initial boiling point of saidoil.

10. The process of claim 1 wherein between and 1,500 standard cubic feetof hydrogen per barrel of feed oil are consumed.

11. The process of claim 1 wherein said catalyst comprises cobalt andmolybdenum.

12. The process ofclaim 1 wherein said catalyst comprises nickel, cobaltand molybdenum.

13. The process of claim 1 wherein the pressure is at least 750 psi.

14. The process ofclaim 1 wherein at least 10 barrels of said oil arecharged per pound of said catalyst.

1. A PROCESS FOR THE HYDRODESULFURIZATION OF AN ASPHALTENECONTAINING OILAT A REACTION TEMPERATURE IN THE RANGE 600* TO 900*F WHEREIN SAID OILAND HYDROGEN ARE PASSED OVER A SULFIDED CATALYST CONTAINING GROUP VI ANDGROUP VIII METALS ON ALUMINA, SAID CATALYST HAVING BEEN SULFIDED BY ANORGANIC SULFUR COMPOUND CONTAINING LESS THAN 8 CARBON ATOMS IN THEABSENCE OF HYDROGEN, ADDING TO SAID CATALYST AMMONIA IN AN AMOUNT TOINCREASE HYDRODESULFURIZATION AS COMPARED TO THE ABSENCE OF SAIDAMMONIA, THE AMMONIA CONCENTRATION IN THE REACTION GASES BEING LOWERTHAN THE AVERAGE HYDROGEN SULFIDE CONCENTRATION IN SAID GASES, ANDINCREMENTALLY INCREASING THE TEMPERATURE WITHIN SAID TEMPERATURE RANGETO COMENSATE FOR LOSS OF CATALYST ACTIVITY DUE TO CATALYST AGING.
 2. Theprocess of claim 1 wherein said organic sulfur compound contains lessthan 6 carbon atoms.
 3. The process of claim 1 wherein said organicsulfur compound contains less than 4 carbon atoms.
 4. The process ofclaim 1 wherein said organic sulfur compound is carbon disulfide.
 5. Theprocess of claim 1 wherein said organic sulfur compound has a boilingpoint below the boiling range of said oil.
 6. The process of claim 1wherein the ratio of said ammonia to said oil is changed during saidprocess.
 7. The process of claim 1 wherein the ratio of said ammonia tosaid oil is changed incrementally prior to said incremental changes intemperature.
 8. The process of claim 1 wherein less than 20 percent ofsaid oil is converted to material boiling below the initial boilingpoint of said oil.
 9. The process of claim 1 wherein less than 10percent of said oil is converted to material boiling below the initialboiling point of said oil.
 10. The process of claim 1 wherein between150 and 1,500 standard cubic feet of hydrogen per barrel of feed oil areconsumed.
 11. The process of claim 1 wherein said catalyst comprisescobalt and molybdenum.
 12. The process of claim 1 wherein said catalystcomprises nickel, cobalt and molybdenum.
 13. The process of claim 1wherein the pressure is at least 750 psi.
 14. The process of claim 1wherein at least 10 barrels of said oil are charged per pound of saidcatalyst.